In a motor control system disclosed in JP-A-9-121595, a technique in which when it is detected-that a temperature of a switching element forming an inverter part increases, a frequency (hereinafter called a carrier frequency) of a carrier signal of a PWM circuit is first decreased and when the temperature increases still, control is performed so as to decrease torque of a motor and an increase in temperature of the inverter part is prevented is disclosed.
On the other hand, in a motor-driven injection molding machine, the practice of periodically opening and closing a movable metal mold driven by a motor with respect to a fixed metal mold is made. In such opening and closing, at the time of closing, the movable metal mold and the fixed metal mold are pressed and the motor becomes a lock state and at the time of opening, the movable metal mold moves and thereby the motor becomes an operation state. That is, the motor repeats the operation and lock states periodically.
In the case that a motor is in the lock state, as described in JP-A-6-38544, a magnitude and a direction of a current flowing through each phase of the motor are fixed, so that a large current flows through one particular switching element forming an inverter part. Therefore, a temperature of the one switching element increases extremely than that of the other switching element.
In such a use, as described in JP-A-9-121595, when the carrier frequency or the torque is decreased after detecting that the temperature of the switching element forming the inverter part increases, there was a problem that power loss of all the switching elements decreases uniformly and it is not efficient and variations in power loss occurring in each the switching element are large.
In addition, when the carrier frequency is decreased, a ripple current of the motor increases and noise generated from the motor increases. When the torque is decreased, there was a problem that torque generated from the motor becomes improper with respect to a drive load.
The present invention is implemented to solve the problems, and an object of the invention is to provide a motor control system for making power loss of each switching element of an inverter part as uniform as possible by passing a d-shaft armature current id through a motor at the time of a lock in a use in which the motor is locked.